The present invention generally relates to a magnetic resonance (MR) imaging apparatus and method for generating a substantially homogenous static magnetic field within a desired spatial region. More particularly the present invention relates to the magnetic resonance imaging of a desired region of a human body (xe2x80x9canatomy of interestxe2x80x9d).
Magnetic resonance imaging relies on the availability of a uniform, homogenous, static magnetic field. In order to image an xe2x80x9canatomy-of-interestxe2x80x9d (e.g. chest, head or female breast etc.) it needs to be placed within the substantially homogenous zone of the static magnetic field generated by the magnet assembly of the overall magnetic resonance imaging apparatus. This zone of the magnetic field which is used in the magnetic resonance imaging will be referred to as the xe2x80x9cimaging-field-zonexe2x80x9d.
Since the quality of the MR image is very dependent upon the degree of homogeneity achieved within the imaging-field-zone, a great deal of time and effort is devoted to the design and optimisation of MR imaging magnets to achieve this. As schematically depicted in FIG. 1a, the main objective of the magnet designer is to design a magnet which generates a static magnetic flux density vector 10;
B(x,y,z)=Bxi+ByJ+Bzkxe2x80x83xe2x80x83(Eq. 1) 
over the imaging-field-zone which is directed substantially along a single axis only (e.g. the z axis in FIG. 1a where B (0,0, z)=Bzk). In Equation 1 above, the vectoral quantities are printed in bold letters and the i, j and k represent xe2x80x98unit vectorxe2x80x99 quantities along the x-axis, y-axis and z-axis, respectively. Furthermore, it can be shown that the Bz component (magnitude of the B vector) can be expressed in the form of;
Bz=B0+B1+B+2+B3+ . . . +Bnxe2x80x83xe2x80x83(Eq.2) 
where the terms xe2x80x9cB1+B+2+B3+ . . . +Bnxe2x80x9d are collectively referred to as xe2x80x9cerror termsxe2x80x9d or xe2x80x9cresidualsxe2x80x9d. The ultimate objective of the magnet designer, during the design process, is to get rid of as many of the xe2x80x9cerror termsxe2x80x9d as possible in order to achieve the ultimate goal of;
Bz=B0xe2x80x83xe2x80x83(Eq.3) 
However, in practice it is not always possible to obtain the same degree of homogeneity within every point of a given imaging field-zone. This will be explained with reference to FIG. 1a which is a schematic, longitudinal section of a prior-art magnet (conventional, six-coil, cylindrical, narrow-bore, axially-long) assembly. A lateral view of the patient is also included. Conventionally, the patient 1 is arranged to lie on a patient-positioner 7 which is parallel to the z-axis of the magnet. The conventional whole body scanner magnet assembly comprises a low temperature chamber 21 operating at around 4xc2x0 K (i.e. liquid helium temperature). Superconductive low temperature coils 20 are arranged within the chamber 21. A thermal radiation shield 22 is arranged around the chamber 21 and around the radiation shield 22 is arranged a high temperature chamber 23 operating at 77K (i.e. liquid nitrogen temperature). Surrounding the high temperature chamber 23 is a vacuum chamber 24. Within the bore of the magnet of the assembly there is provided an arrangement 26 of room temperature shims, gradient coils and radio frequency (RF) coils.
The general purpose, conventional, whole-body MR imaging systems are designed with the objective of being able to image effectively all parts of the human anatomy by using the centre of the thorax as reference-point 28 (i.e. x=0, y=0 and z=0). They are designed as axially symmetric structures. The magnet design process involves the selection of the number of superconductive coils 20, their axial and radial locations, the magnitude of the current flowing through the coils, the number of turns, etc. When all these parameters are xe2x80x98optimallyxe2x80x99 selected, then a substantially homogenous magnetic field within an imaging-field-zone, which is centred at the geometrical centre 28 of the magnet assembly, may be generated. However, although the magnet designer can achieve theoretically possible maximum homogeneity (i.e. Bz=B0) at the geometrical centre 28 of the magnet, inherently the same maximum homogeneity can not be achieved at other spatial locations. In other words, as schematically illustrated in FIGS. 2a and 2b, although the field is xe2x80x9cpurexe2x80x9d at the geometrical centre 28 of the magnet, the degree of homogeneity decreases gradually at increasing distances away from the geometrical centre 28. Figuratively speaking, for example the homogeneity calculated in a, say, 10 cm diameter imaging-field-zone is much better that the homogeneity calculated for much larger imaging-field-zones (e.g. say 20 cm, 30 cm etc, diameter zones). This is schematically depicted in FIGS. 2a and 2b where the most homogenous zone within the magnet""s bore 27 is zone 12a (i.e. zone with the smallest diameter). The homogeneity calculated for the zones 12b, 12c and 12d is increasingly much lower that the homogeneity calculated for zone 12a. 
Now, let us assume the anatomy-of-interest which needs to be imaged, with a prior art, conventional whole body MR scanner, is female breast. More specifically, as depicted in FIGS. 1a, 1b and 1c, the anatomy-of-interest 2 comprises the left-breast 2a and the right breast 2b of the patient 1. It can be seen that the centre 6 of the anatomy-of-interest is not at the geometrical centre 28 of the magnet assembly. This means that the area 11 of the anatomy-of-interest does not lie within the available most homogenous zones of the magnet""s imaging-field. Referring now to FIGS. 1b and 1c, if one attempts to position the centre of the breast 6 at the geometrical centre of the whole body magnet 28 by lifting the patient upwards, in majority of the cases, this is not permitted by the narrower bore size of the magnet. As illustrated in FIG. 1c, this would not be possible since the arms 4a and 4b of the patient 1 would be required to lie within the regions provided for the room temperature shim coils, gradient coils and RF coils 26 in a conventional whole body scanner. For this reason, one could argue that the breast images obtained by a whole body scanner is not always optimal, since they are not always imaged by making use of the most homogeneous magnetic field available.
Unfortunately, at present, the clinical application of breast MR imaging is restricted to conventional, general purpose, whole body scanners (e.g. figure 1a), and the breast referral cases have to compete for scanner time with many other cases of referrals. The design of the conventional xe2x80x98whole bodyxe2x80x99 scanners tend to encapsulate the patient and only permit limited access to patient by the health care professionals and make the task of performing interventional procedures more complicated and difficult. Interventional radiologists and surgeons require:
(a) totally xe2x80x98open-accessxe2x80x99 to breast and patient;
(b) easy xe2x80x98image-guidedxe2x80x99 diagnostic procedure (fine needle aspiration, core biopsy etc.);
(c) easy xe2x80x98image-guidedxe2x80x99 surgical treatment (e.g. lumpectomy) and other therapeutic procedures (e.g. tumour ablation using laser, rf, microwave, focused ultrasound energies, etc.).
The whole body cylindrical magnet based MR scanners (see FIG. 1a) does not allow the direct interactive placements of interventional devices (e.g. biopsy needles and optical/laser fibres used for thermal ablation of tumours) into the breast. They require the withdrawal of the patient from the bore of the magnet in order to gain access to the breast. This together with the possible movement of the patient may result in the imprecise insertion and localisation of the interventional devices.
Furthermore, in addition to the problem of patient inaccessibility within the bore of the conventional whole body scanner, the claustrophobic nature of the design due to patient encapsulation is also a factor affecting patient-acceptance rate of whole body scanners. All of these limit the widespread use and the exploitation of the full potential MR modality in the overall management of patients, in particular the patients with breast cancer.
One attempt to reduce the problem of patient encapsulation is described in U.S. Pat. No. 4,701,736, the content of which is hereby incorporated by reference. This patent discloses a so-called planar magnet design in which the axial length of the magnet assembly is reduced. This reduction in axial length reduces patient encapsulation. However, here, the motivation is to replace the conventional, solenoidal, axially long magnet with a shorter magnet but the narrow bore size still kept the same. This patent also discloses magnetic arrangements where the imaging-field-zone is projected outside the bore of the magnet to avoid any encapsulation at all.
A similar technique is disclosed in UK Patent No. 2285313 and U.S. Pat. No. 5,596,303, the contents of which are hereby incorporated by reference. In these patents an improved magnet assembly is provided which uses close-in high temperature superconducting correction coils to provide the required homogenous field within the bore of the magnet or extending beyond the bore of the magnet. This arrangement provides a similar benefit of reduced patient encapsulation as provided by the arrangement of the U.S. Pat. No. 4,701,736.
The problem of patient encapsulation is also considered in International patent publication WO 94/06034. The arrangement discloses a frustoconical magnetic resonance imaging magnet assembly which provides a large opening to allow a healthcare professional access to the patient. Further, the patient can be inclined relatively within the frustoconical magnetic assembly to further enhance patient accessibility. Although this reduces the degree of patient encapsulation, it requires a large magnetic assembly having a large axial length.
Further attempt to reduce patient encapsulation while still providing whole body scanning is disclosed in International patent publication WO 98/00726. In this arrangement a toroidal shaped magnetic assembly is used to form a sequence of cross-section images of an object and to form a complete image of the object by scanning.
The present invention provides an improved magnetic resonance imaging technique for imaging a region of a patient which requires a minimum patient encapsulation and which provides a large bore, axially short magnet which enables the patient and the magnet to be relatively positioned in three dimensions and/or in relative inclination to enable the desired region of a patient to be positioned at the most homogenous region of the magnetic field within the bore of the magnet. The positioning of the centre of the anatomy of interest to be imaged of the patient at the centre of the region of homogeneity i.e. at the region of highest homogeneity, ensures an image of the highest fidelity will be produced.
Thus in accordance with the first aspect of the present invention a magnetic resonance imaging magnet assembly has a bore large enough to enable any part of the patient to be placed on the axis of the bore at which the imaging-field-zone is centred and has the highest homogeneity. In the present invention either the patient can be manoeuvred in three dimensions within the bore, or the magnet assembly can be moved three dimensionally relative to the patient to provide the relative positioning. An added advantage of the large bore of the magnet assembly is that there is a minimum patient encapsulation and healthcare professionals are provided with good access to the region of the patient being imaged.
In the prior art axially short magnet based magnetic resonance imaging systems, although there is some improvement to the accessibility of the patient, the size of the bore is insufficient to enable all regions of the patient to be imaged to be placed within the most homogeneous region. In this aspect of the present invention, because of the large bore provided by the magnet assembly, it is possible to image any body part of the patient within the most homogeneous zone of the imaging-field-zone.
In one embodiment of the present invention, the bore of the magnet has a diameter which is preferably larger than twice the length measured between the iso-centre of the breast of an average height female and the tip of her head. The bore diameter is, in one preferred embodiment, at least 110 cms. In one preferred embodiment the axial length of the bore is less than 40 cms. Thus in one preferred embodiment of the present invention the ratio of the diameter to the axial length is at least 1.5.
In another aspect of the present invention there is provided a magnetic resonance imaging technique in which a patient is positioned in the bore of a large bore magnet so that the patient is inclined relative to the axis of the magnet assembly so that a desired slice of the anatomy-of-interest of the patient that is to be imaged is positioned in a prefered imaging plane within the imaging-field-zone of the magnet assembly. The ability to relatively incline the patient and the magnet assembly enables a health care professional to obtain an image of a desired slice of the anatomy-of-interest of high quality because of positioning of the slice of the anatomy-of-interest an imaging-plane of highest homogeneity within the imaging-field-zone.
Thus in accordance with this aspect of the present invention, the large size of the bore enables a patient to be imaged in an inclined position within the bore of the magnet to enable a healthcare professional to select the direction of the imaging-plane to coincident with the desired slice of the anatomy-of-interest to be imaged.
In a preferred embodiment of the present invention a patient is placed in the bore of the magnet and then relative rotation between the patient and the magnet assembly takes place such that the patient is relatively rotated in 3D (three dimensions) in the bore about a pivot point on the axis at the geometric centre of the bore, whilst maintaining the anatomy of the interest of the patient in the imaging field zone of the magnet assembly. Thus, in accordance with this embodiment of the present invention, a healthcare professional can select the angle of inclination and in one embodiment of the present invention, more than one image can be taken at different angles of inclination with the anatomy-of-interest of the patient being kept centred on the centre of the imaging-field-zone having the highest homogeneity.
Thus in accordance with this aspect of the present invention, the relative inclination of the patient and the magnet assembly provides the benefit of enabling the healthcare professional to select the angle of the image plane in the anatomy-of-interest of the patient, and provides the healthcare professional with a greater accessibility to the region of the patient being imaged.